1. Field of the Invention
This invention relates to expandable thermoplastic resin particles. More particularly, it relates to expandable thermoplastic resin particles which can produce molded vessels having the advantages which prevent oil or fat in oily or fat containing foods such as instant noodle or fried chicken or regular coffee in the vessels from exuding outwards through the fusing surface between foamed particles on vessel wall, prevent water from exuding out of drain pan (saucer) used for home air-conditioners, and/or prevent ice-water from exuding out of portable simplified iceboxes.
2. Description of the Prior Art
Expandable thermoplastic resin particles are prepared by impregnating, for example, polystyrene resin particles with a small amount of a volatile fatty hydrocarbon, e.g. n-pentane, which only slightly swells the particles in an aqueous suspension, or by impregnating the particles with a blowing agent such as butane or propane in usual gas form in an aqueous suspension containing a small amount of a slovent such as toluene or cyclohexane having solubility to polystyrene resin particles. Thus obtained expandable thermoplastic resin particles are used as materials for preparing thermoplastic foamed molded articles. To obtain thermoplastic foamed molded articles economically and industrially, expandable thermoplastic resin particles are firstly pre-expanded and the resulting pre-expanded particles are charged into a perforated mold cavity of a molding machine, where the particles are heated over their softening point by injecting pressured steam into the perforation of the perforated mold cavity to unite each pre-expanded particle by fusing together, whereby a molded article as the cavity being obtained.
In case expandable thermoplastic resin particles used for such object contain a substance of good affinity to the thermoplastic resin such as n-pentane, they have a tendency of coagulating the particles during the pre-expanding process because of lowering of the thermal stability. The coagulated particles block a particles conveying pipe or a mold cavity charging hole, disturbing a molding process. Accordingly, expandable thermoplastic resin particles of no coagulation in pre-expanding are required. Thus, to cover the surfaces of expandable thermoplastic resin particles with, for example, metal soap, talc powder, or wax in advance has been well known. But, these surface covering agents, on the other hand, have a tendency to retard fusing together of the resin particles in molding. It is also a known method to add materials which prevent coagulation prior to molding and do not retard fusing together in molding. Thus obtained known foamed molded articles, however, have a fusing together surface of imperfect uniting with opening of fine capillaries in cutting it, even if the fusing together state of each expandable polymer particle is satisfactory, that is, no surface of each particle appears on the cut surface to 100% of the degree of fusing together. It can be confirmed, for example, by noticing that an aqueous solution of a dyestuff containing a surfactant put into the molded article exuded out through the article. More concretely, when a molded article obtained by the known molding process using conventional expandable styrene polymer particles, e.g. a cup, was allowed to stand after putting a coffee solution containing 0.01 w/w % of sodium alkylbenzenesulfonate, after about 10 min the coffee solution notably exuded out of the cup side-wall through the clearance between foamed particles, even if the cup was a normal molded fusing together state. Although these cups had no difficulty in practical use as drinking cups for normal coffee or Coke, they were unsuitable for vessels for oily foods, for example, foods containing salad oil, tallow, or lard such as doughnut, hamburger, fried chicken, or margarin, because in such foods stored for long periods oil or fat exuded gradually out of vessel wall. Further, when instant noodle mixed with curry was stored after put into these vessels, the yellow color in curry exuded out of vessel wall with staining the vessels, markedly reducing the commercial value as instant noodle with vessel. Further, since a drain pan of foamed molded thermoplastic for a home small type air-conditioner has drain water exudation, usually a pan applied coating forming paraffin or resin film on the molded surface is used. Also, in a simplified ice-box portable for trip of foamed molded thermoplastic ice-water exudes in long hours, lowering the commercial value.
On the other hand, there has been a proposal of covering expandable thermoplastic resin particles with phosphoric acid ester containing perfluoroalkyl group (Japanese Patent Laid-Open Sho No. 59-41339). However, the resin particles had a difficulty to fusing together when molded, whereby a problem of lowering productivity was resulted, requiring prolongation of the heating time for molding.
Further, by the formation of fused film on the surface of foamed molded articles by the use of a particular mold or under the heating condition at an elevated temperature in molding expandable styrene polymer, oil or fat exudation can be prevented. However, in conventional expandable polystyrene particles, the above method causes the foamed articIe to shrink with fusing because of the low thermal stability, being difficult to obtain a beautiful molded article having an appearance of good commercial value. In addition, the marked prolongation of molding cycle time at an elevated temperature reduces the productivity. The cups obtained by such method are lower in bending strength than those obtained by a conventional method, being nearly unusable practically because of the brittle property.